The invention relates to an image processing method of an X-ray CT, an X-ray CT, and an X-ray CT image-taking recording medium, especially, to a technique for reducing a false image or artifact by absorption or dispersion of X-rays by an X-ray high-absorber, such as metal.
Generally, as shown in FIG. 8(a), an X-ray CT includes an X-ray tube 51 for generating an X-ray beam B and an X-ray detector 52 for detecting X-rays. The X-ray tube 51 and the X-ray detector 52 are disposed to sandwich an object P to be tested, and the X-ray tube 51 irradiates the X-ray beam B to the object P to be tested to take images while rotating around an axis Z of the object P (in a direction extending vertically with respect to a sheet surface in FIG. 8(a)). Generally, the X-ray detector 52 includes a plurality of detecting elements CHi (i=1, 2 . . . , nxe2x88x921, n wherein n is a natural number), and the detecting elements CHi are disposed in a fan shape every fine angle. (FIG. 8(a))
In a conventional image processing method, X-rays are irradiated from the circumference of the object P to be tested to obtain X-ray transmission data f(P) on a projection plane U (FIG. 8(b)). Incidentally, in the present specification, the X-ray irradiation is referred to as xe2x80x9cthe object P to be tested is projected in an irradiation direction of the X-ray beam Bxe2x80x9d (hereinafter, also abbreviated to xe2x80x9cthe object P to be tested is projectedxe2x80x9d). The X-ray transmission data f(P) obtained on the projection plane U at this time is also referred to as xe2x80x9cmeasured projection data f(P)xe2x80x9d. Further, the measured projection data f(P) on the projection plane F is subjected to a reconstruction process, such as filtering and backprojection, to obtain an original image G[f(P)] on an image plane V (FIG. 8(c)). In the present specification, the filtering is defined such that a convolution integral calculus (superposition integral calculus) is carried out by using a convolution kernel function. Also, the method where the projection data is subjected to the reconstruction process, such as filtering and backprojection, is generally known as a filtered backprojection (hereinafter referred to as FBP) method. Incidentally, since the FBP method is one of representative reconstruction processing methods, explanation thereof is omitted. By the way, derivation of the projection data of an image by using a mathematical algorithm is called as xe2x80x9cforward projecting the image in an irradiation direction of X-raysxe2x80x9d (hereinafter, if applicable, abbreviated as xe2x80x9cforward projecting the imagexe2x80x9d).
Incidentally, a row of the detecting elements CHi and a row of angles xcex8 in an irradiation direction of X-ray beam B are shown in a horizontal axial direction and in a vertical axial direction on the projection plane U, respectively. Also, hatching areas in the projection plane U and the image plane V are shown illustratively. Further, the projection plane U and image plane V show areas. Based on the above, the following explanation is provided.
Incidentally, in case a certain object is represented by xe2x80x9cPxe2x80x9d, the projection of the object P is indicated by f(P); in case a fault image is represented by xe2x80x9cxcex1xe2x80x9d, the forward projection of the fault image xcex1 is indicated by F(xcex1); and in case certain projection data is represented by xe2x80x9cxcex2xe2x80x9d, the fault image obtained by filtered and backprojection of the projection data xcex2, i.e. the image reconstituted by the FBP method is indicated by B[H(xcex2)] or G(xcex2). At this time, it is assumed that an operation of the backprojection is represented by xe2x80x9cBxe2x80x9d; an operation of the filtering is represented by xe2x80x9cHxe2x80x9d; and an operator of the backprojection and filtering is represented by xe2x80x9cGxe2x80x9d. Also, in order to make distinction between the projection of the object to be tested and the forward projection of the image, the symbols with respect to the projection and the forward projection are represented by xe2x80x9cfxe2x80x9d and xe2x80x9cFxe2x80x9d. Further, since there is a measurement error, in case the projection data xcex2 is measured, even if the fault image obtained by the FBP method of the measured projection data xcex2 is forward projected, the forward projected fault image does not return to exactly the same data as the originally measured projection data xcex2. Therefore, in the present specification, assuming that xcex2=F[G(xcex2)] does not hold, the following explanation is made. Incidentally, the projection data (including also the measured projection data f(P)) and the image (including also the measured fault image G[f(P)]) have numeral weights, different from the projection plane F and the fault plane G, in other words, they are dealt as pixel values in the following explanations of the present specification.
However, in case of the conventional image process method, there are the following problems.
In detail, when a reconstruction of the image is carried out by taking an image of an object to be tested, false images are generated. Especially, in case of taking an image of an object to be tested including a high-absorber consisting of a metal or the like, the false images generated at and around the high-absorber become conspicuous due to absorption or dispersion by the high-absorber. Hereinafter, in the present specification, the above-explained false image is called as xe2x80x9cartifactxe2x80x9d. In the false images, especially, there are known streak artifacts where radially striped patterns are generated around the high-absorber, and shading artifacts which are generated at portions sandwiched by a plurality of high-absorbers. In order to reduce the artifacts including the above-mentioned artifacts, various methods have been proposed. As representative reducing methods, there are mentioned an iterative reconstruction and reprojection (hereinafter referred to as xe2x80x9cIRRxe2x80x9d) method, and algebraic reconstruction technique/expectation and maximization (hereinafter referred to as xe2x80x9cART/EMxe2x80x9d) method.
First, the IRR method is explained with reference to FIG. 9. In the IRR method, an original fault image G[f(P)] on the fault plane V obtained by a conventional image processing method is again forward projected by a mathematical algorithm in an irradiation direction of X-rays to obtain forward projection data F(G[f(P)]) on the projection plane U. Then, an operator sets a portion corresponding to a high-absorber as a high-absorber area M on the image plane V with reference to the original fault image G[f(P)] on the fault plane G ((a) in FIG. 9). Incidentally, the high-absorber area M is a closed area. A portion L where X-rays pass through the high-absorber area is formed in the projection plane U. A pixel value of the measured projection data f(P) with respect to the portion L where X-rays pass through the high-absorber area is replaced by a pixel value of the forward projection data F(G[f(P)]) or a pixel value derived from the forward projection data F(G[f(P)]) to correct the measured projection data f(P) and obtain a corrected projection data F(P1). It should be noted that the forward projection data F(G[f(P)]) is different from the corrected projection data F(P1) ((b) in FIG. 9) Further, reconstruction of the corrected projection data F(P1) is carried out again to obtain a corrected fault image G[F(P1)] on the image plane V. ((c) in FIG. 9)
Incidentally, xe2x80x9cthe pixel value of the measured projection data f(P) of the portion L where X-rays pass through the high-absorber area is replaced by the pixel value of correcting forward projection data F(G[f(P)])xe2x80x9d means that the corrected projection data F(P1) is obtained by following equations (1) and (2).
In the portion where X-rays do not pass through the high-absorber area:
F(P1)=f(P)xe2x80x83xe2x80x83(1)
In the portion L where X-rays pass through the high-absorber area:
F(P1)=F(G[f(P)])xe2x80x83xe2x80x83(2)
Incidentally, xe2x80x9cthe pixel value of the measured projection data f(P) of the portion L where X-rays pass through the high-absorber area is replaced by the pixel value derived from the correcting forward projection data F(G[f(P)])xe2x80x9d can be obtained by equations (3) and (4) mentioned below.
In the portion where X-rays do not pass through the high-absorber area:
F(P1)=f(P)xe2x80x83xe2x80x83(3)
In the portion L where X-rays pass through the high-absorber area:
F(P1)=xcex1xc3x97F(G[f(P)])xe2x80x83xe2x80x83(4)
In other words, the pixel value of the measured projection data f(P) of the portion L where X-rays pass through the high-absorber area is replaced by a value obtained by multiplying the forward projection data F(G[f(P)]) of the portion L where X-rays pass through the high-absorber area at predetermined times (xcex1 times).
By the above-stated method, in the portion L through which X-rays pass in the high absorber area, the pixel value of the forward projection data F(G[f(P)]) or the pixel value derived from the forward projection data F(G[f(P)]) passing through the high-absorber area is utilized, instead of the pixel value of the measured projection data f(P). Therefore, in the portion where X-rays do not pass through the high-absorber area, the pixel value of the measured projection data f(P) is left as it is, while in the portion L where X-rays pass through the high-absorber area, data obtained by subjecting to the filtered, backprojection (reconstruction by the FBP method) process and the forward projection is replaced therewith. Here, in the portion L where X-rays passes through the high-absorber area, the correcting forward projection data F(G[f(P)]) is more accurate than the measured projection data f(P) as the data. Therefore, in the corrected projection data F(P1), the forward projection data F(G[f(P)]) to reduce the artifact portion is used. However, in the IRR method, since it is required that the correction data should be very accurate, the effects are limited and reduction effects are poor. On the other hand, as a method where the reduction effect of the artifact is high, there is an ART/EM method.
Next, the ART/EM method is explained with reference to FIGS. 10 and 11. FIG. 10 is a drawing showing a process for obtaining an estimated image b1 by overwriting an initial image b0 one time; and FIG. 11 is a drawing showing a process for obtaining an estimated image bk+1 by further overwriting or amending, one time, an estimated image bk obtained by overwriting the initial image b0 plural times (in this case, k times). Incidentally, b0 in FIG. 10 is an estimated image initialized by an arbitrary positive value, and b1 is an initial image obtained by overwriting the initial image b0 one time. Also, bk in FIG. 11 is an estimated image obtained by overwriting the estimated image b0 k times, and bk+1 is an estimated image obtained by overwriting the estimated image b0 (k+1) times.
As shown in FIG. 10, in the specific example of the ART/EM method, the initial image b0 is set. The initial image b0 can be set with an arbitrary positive value by an operator. For example, the pixel values of X-rays on the whole area may be the same. Then, the operator sets a portion corresponding to the high-absorber as a high-absorber area M on the image plane V referring to a measured fault image G[f(P)] on the image plane V ((a) in FIG. 10) Then, a portion L where X-rays pass through the high-absorber area is formed on the projection plane U. The initial image b0 is forward projected to obtain estimated projection data F(b0) on the projection plane U. Then, the obtained estimated projection data F(b0) is compared with the measured projection data f(P) ((b)in FIG. 10). When compared, the portion L where X-rays pass through the high-absorber area is disregarded and not compared. In other words, with respect to a portion where X-rays do not pass through the high-absorber area, the estimated projection data F(b0) is compared with the measured projection data f(P). This is also applied to the overwritten, described later, wherein overwriting of the estimated image is carried out only for the portion where the high-absorber does not exist.
Incidentally, in the ART method, the measured projection data f(P) and the estimated projection data F(b0) are compared, and a difference of both data is backprojected to obtain a comparison reference image. With the comparison reference image, the initial image b0 is overwritten. On the other hand, in the EM method, a ratio of the measured projection data f(P) with respect to the estimated projection data F(b0) is backprojected to obtain a comparison reference image. In the same manner, with the comparison reference image, the estimated image b0 is overwritten. By the above-stated method, the initial image overwritten one time becomes an estimated image b1 ((c) in FIG. 10). With reference to the pixel values of the portion L where X-rays pass through the high-absorber area, as described before, comparison of both projection data and overwriting are not carried out and disregarded. Thus, the data of the pixel values of the portion L has no definition nor distinction.
In the same manner as in the overwriting method of the initial image b0, the estimated image f(b1) is compared with the measured projection data f(P) to overwrite the estimated image b1. Thereafter, in the same manner, the initial image is overwritten plural times to obtain an intended fault image. More specifically, when the overwriting of the estimated image bx is carried out, as shown in FIG. 11, the estimated image f(bx) is compared with the measured projection data f(P) to derive or obtain an estimated image bk+1.
In the image obtained by the above-described methods, higher effects are obtained for reducing the artifact portion when compared with that of the IRR method. Especially, the reduction effects of the streak artifact are conspicuous in the ART/EM method. However, in the ART/EM method, as described before, since the portion L where X-rays pass through the high-absorber area is disregarded when the comparison between the projection data and overwriting is carried out, data regarding the high-absorber is not defined. Therefore, there is a defect wherein the shapes of the high-absorber itself and its circumference are not reproduced. Also, in case the number of times by which the overwritings of the estimated images are carried out, i.e. repetition times, is small, it only an image having a low contrast can be obtained, which results in a poor resolution. In order to obtain a high contrast fault image, the number of times by which the estimated images are renewed, i.e. repetition times, should be increased.
In view of the above problems, the present invention has been made and an object of the invention is to provide an image processing method of X-ray CT, X-ray CT and X-ray CT image-taking recording medium for reducing artifact on a fault image caused by absorption or dispersion by a high-absorber.
Further objects and advantages of the invention will be apparent from the following description of the invention.
In the ART/EM method, as described above, since the portion where X-rays pass through the high-absorber area is disregarded, shapes of the high-absorber itself and the circumference thereof can not be reproduced. Therefore, the ART/EM method considering the portion relating to the high-absorber is desired. However, in case the data of the portion where X-rays pass through the high-absorber area is used as it is, reduction effects of the artifact can not be obtained. Therefore, there is considered an ART/EM method wherein weighting is carried out according to a length through which X-rays pass in the high-absorber area (hereinafter, abbreviated as xe2x80x9cweighted ART/EM methodxe2x80x9d).
Also, in the weighted ART/EM method, since the portion where X-rays pass through the high-absorber is considered, the reduction effect of the artifact is reduced when the repetition times are increased. In other words, when the repetition times are small, although the artifact portion is reduced, only low contrast image can be obtained. On the contrary, when the repetition times are large, although a high contrast image can be obtained, the reduction effect of the artifact is lowered. However, in the IRR method, when measured projection data of the portion where X-rays pass through the high-absorber area is replaced by the forward projection data with reduced artifact obtained by the weighted ART/EM method when the number of repetition times is small to thereby correct the measured projection data, and then a reconstruction process is further carried out by the FBP method or ART/EM method to obtain a high contrast image with reduced artifact.
Thus, the present inventors had an idea such that in addition to the combination of the IRR method and the weighted ART/EM method, further, when a reconstruction process is carried out by the FBP method or ART/EM method, a corrected image with reduced artifact can be obtained.
The present invention based on the above-described knowledge and information has the following structure.
In an image processing method according to a first aspect of the invention, a processing method obtains a fault image by reconstructing image data obtained when an image is taken by an X-ray CT. The image processing method includes (1) an image-taking process wherein measured projection data can be obtained by irradiating X-rays from a circumference of an object to be tested and detecting the X-rays passing through the object to be tested; (2) a first image reconstructing process for subjecting the measured projection data obtained in the image-taking process to filtering and then backprojecting to thereby reconstruct an original image; (3) an initial image setting process for initializing or setting an estimated image with an arbitrary positive value; (4) a high-absorber area setting process for setting a high-absorber area based on the original fault image derived from the first image reconstructing process; (5) an estimated projection data derivation process for deriving estimated projection data by forward projecting the estimated image in an X-ray irradiation direction; (6) a comparison reference image deriving process for deriving or obtaining a comparison reference image by backprojecting a difference or a ratio between the estimated projection data and the measured projection data; (7) a weighted comparison reference image deriving process for deriving or providing a weighted comparison reference image weighted such that as a path through which X-rays pass in the high-absorber area becomes longer, the respective pixel values of the comparison reference image become smaller; (8) an estimated image overwriting process for overwriting the estimated image by the weighted comparison reference image; (9) a repeating operation process for overwriting an estimated image where an artifact is reduced by repeatedly carrying out, one time or plural times, the above-stated respective processes from (5) to (8) according to degrees of the artifact appearing on the estimated image overwritten at the estimated image overwriting process; (10) an overwritten estimated projection data deriving process for deriving or providing overwritten estimated projection data through the forward projection of the estimated image overwritten at the repeating operation process; (11) a measured projection data correction process for replacing the measured projection data of the portion where X-rays pass through the high-absorber area with the data according to the overwritten estimated projection data to correct the measured projection data; and (12) a second image reconstructing process for reconstructing the image of the corrected measured projection data to derive the fault image.
In an X-ray CT according to a second aspect of the invention, an X-ray CT obtains a fault image by reconstructing image data obtained when an image is taken by the X-ray CT. The X-ray CT includes (a) an X-ray irradiation device for irradiating X-rays from the circumference of an object to be tested; (b) an X-ray detecting device for obtaining measured projection data by detecting X-rays passing through the object to be tested by irradiation of X-rays by the X-ray irradiation device; (c) a first image reconstructing device for reconstructing the measured fault image by subjecting the measured projection data obtained by the X-ray detecting device to filtering and then backprojecting the filtered measured projection data; (d) an estimated image setting device for initializing or setting an estimated image with an arbitrary positive value; (e) a high-absorber area setting device for setting a high-absorber area based on the measured original image derived from the first image reconstructing device; (f) an estimated projection data deriving device for deriving estimated projection data by forward projecting the estimated image in the X-ray irradiation direction; (g) a comparison reference image deriving device for deriving a comparison reference image by backprojecting a difference or a ratio between the estimated projection data and the measured projection data; (h) a weighted comparison reference image deriving device for deriving a weighted comparison reference image weighted such that the pixel values of the comparison reference image become smaller as a path through which X-rays pass in the high-absorber area become longer; (i) an estimated image overwriting device for overwriting the estimated image by the weighted comparison reference image; (j) a repeating operation device for overwriting to an estimated image wherein the artifact is reduced by repeatedly carrying out processes, one time or plural times, by the respective devices from (f) to (i) according to a degree of the artifact appearing on the estimated image overwriting by the estimated image overwriting device; (k) a overwritten estimated projection data deriving device for deriving the overwritten estimated projection data by forward projecting the estimated image overwritten by the repeating operation device; (1) a measured projection data correcting device for correcting the measured projection data by replacing the measured projection data of the portion where X-rays pass through the high-absorber area by the data according to the overwritten estimated projection data; and (m) a second image reconstructing device for deriving a fault image by reconstructing an image of the corrected measured projection data.
In an X-ray CT image-taking recording medium according to a third aspect of the invention, a computer-readable X-ray CT image-taking recording medium is obtained, wherein a program for executing the image process method, mentioned in the first aspect of the invention, and read by a computer is recorded.
A function of the invention described in the first aspect is explained.
According to the image process method of the invention, by setting of a high-absorber area, an estimated image considering the high-absorber area can be obtained. Through comparison of the measured projection data and estimated projection data obtained by the forward projection of the estimated image in the X-ray irradiation direction, a comparison reference image is derived based on the backprojection of a difference or a ratio between the data mentioned above. The comparison reference image is weighted according to the path, i.e. length, through which X-rays pass in the high-absorber area. Thus, the weighted comparison reference image is overwritten to an estimated image having the reduced artifact caused by dispersion or reflection of the high-absorber area. The overwritten estimated image of the high-absorber area and overwritten estimated projection data obtained by forward projecting the overwritten estimated image in the X-ray irradiation direction have reduced artifact. The measured projection data at the portion where X-rays pass through the high-absorber area is replaced by the data according to the overwritten estimated projection data to reconstruct the measured projection data through the correction thereof. Thus, a corrected image having a high contrast and reduced artifact can be obtained.
According to the invention described in the second aspect, the method of the invention of the first aspect can be favorably carried out, so that a corrected image having a reduced artifact and a high contrast can be obtained.
According to the invention described in the third aspect, the method of the invention described in the first aspect is carried out by a computer to thereby obtain a corrected image having a reduced artifact and a high contrast.